The subject matter described herein relates to hydrocarbon traps used in automotive exhaust systems and to methods of operating such traps to improve their performance in trapping cold start engine emissions.
In recent years, considerable efforts have been made to reduce the level of hydrocarbon (HC) emissions from vehicle engines to meet increasingly stringent emissions standards. Conventional exhaust treatment catalysts such as three-way catalysts (TWC) achieve conversion of hydrocarbons to water and carbon dioxide and help prevent the exit of unburned or partially burned hydrocarbon emissions from a vehicle. Such three-way catalysts are effective to convert over 99% of hydrocarbon emissions in engine exhaust during normal engine operation after warm-up. However, hydrocarbon emissions are high during cold starting of the engine and enter the vehicle's exhaust system before the latent heat of the exhaust gases allows the catalyst to become active, i.e., before the catalyst has reached its “light-off” temperature, defined as the temperature at which the three-way catalyst is effective to convert at least 50% of the unburned hydrocarbon emissions.
Hydrocarbon traps have been developed for reducing emissions during cold-start by trapping/adsorbing hydrocarbon (HC) emissions at low temperatures and releasing/desorbing them from the trap once sufficiently elevated temperatures are reached for oxidation over a catalyst, such as a three-way catalyst. Currently, zeolites are the most widely used adsorption materials for hydrocarbon traps due to their unique cage-like lattice structures. In a conventional hydrocarbon trap design, trapping material such as a zeolite is coated on the walls of, for example, a honeycomb substrate having gas flow passages or channels therethrough. Three-way catalyst is washcoated over the hydrocarbon trap material. As exhaust gases flow through the trap, hydrocarbon emissions are adsorbed by the zeolite material during cold start and are ideally released when the three-way catalyst is warmed to its light-off temperature from the heat in the exhaust gases.
However one major obstacle is that hydrocarbon storage materials such as zeolites normally cannot retain all of the hydrocarbons until the light-off temperature for the three-way catalyst is reached. Typically, on cold starting, more than 50% of the trapped hydrocarbons have already desorbed from the trapping material and have passed through the trap before light-off temperature has been reached. These desorbed hydrocarbons have no chance of being catalytically converted prior to exiting the vehicle exhaust system. In order for a high percentage of hydrocarbons to be converted, the three-way catalyst should be fully active as the hydrocarbons are desorbed from the zeolite trapping materials. This temperature mismatch between a conventional layer configuration design of zeolite and three-way catalyst results in overall poor hydrocarbon trap performance during cold starts.
The art has attempted to improve upon the performance of trapping materials and three-way catalysts with the objective to adsorb more hydrocarbons at low temperatures, delay the release of adsorbed hydrocarbons until higher temperatures are reached, and develop three-way catalysts that have lower light-off temperatures. A further complication is that after the materials in conventional hydrocarbon traps have aged through repeated use, the hydrocarbon trapping materials tend to absorb fewer hydrocarbons and release them at lower temperatures, and the three-way catalysts tend to require higher temperatures to reach light-off.
Accordingly, there remains a need in this art for hydrocarbon traps that are better able to achieve the desired goals of reducing hydrocarbon emissions from vehicle exhaust systems during cold start ups.